The present invention relates to methods for evaluation of semiconductor processing and, more particularly, to a method for evaluating the exposure pattern provided by a photolithography system. A major objective of the present invention is to provide a method providing for enhanced evaluation photolithographic line uniformity.
Much of recent technological progress is associated with the increasing integrated circuit miniaturization afforded by advances in semiconductor processing. The degree of miniaturization can be quantified by a line width or feature size. In this vein, one micron technology provides for denser, more sophisticated and faster integrated circuits than does two micron technology. Still further advances are provided by submicron processing technology.
Each line width reduction places further demands for precision and uniformity on the photolithographic system used to define the features of the integrated circuit. In photolithography, a photoresist is exposed to a pattern of light. The light chemically alters the exposed portions of the photoresist, while the unexposed portions remain unchanged. The chemical alteration allows the photoresist to be selectively removed, leaving only the unexposed portion (of a positive photoresist) or only the exposed portion (of the more common negative photoresist).
Typically, the photoresist itself is not of direct interest. However, underlying material of interest can be exposed by the removal of photoresist. The exposed underlying material can be processed, while material underlying remaining photoresist is protected. The resulting patterned processing is used to define circuit features. These features can be, for example, conductors, semiconductors, and dielectrics.
In many cases, the photolithographic pattern is imposed by shining a nominally uniform light source through a patterned mask so that the mask pattern is projected on a photo-resist coated wafer. Typically, a semiconductor wafer is processed to yield a multitude of identical integrated circuits. Accordingly, a stepper can maneuver a wafer below the mask so that its pattern can be imposed at many predetermined locations on a wafer.
Diffraction of light by a mask can limit photolithographic resolution. Accordingly, direct write systems can be used. For highest resolution, higher frequency illumination is employed. Electron beam (E-beam) systems, accordingly are available for submicron processing. Particle accelerators are used as direct write beam sources for the highest resolutions currently available. In both the mask-based approach and the direct write approach, a repeatable photolithographic pattern is imposed. In the mask-based approach, the pattern is imposed spatially, and in the direct-write approach, the pattern is imposed temporally.
It is, of course, critical that the light pattern used to define these features be applied with precise localization and intensity. For example, a set of parallel conductive traces should be characterized by constant line widths. Otherwise, some of the lines could: fail to conduct; exhibit higher than acceptable resistances; be subject to unacceptable cross talk; short circuit; or be more prone to break down with repeated use.
To avoid considerable waste of time, money and resources, photolithographic systems are evaluated either in response to detected defects or in anticipation of potential defects. A gross evaluation is available by quantifying the number of defective devices yielded by a system. By examining a completed device microscopically, the location and nature of defects can sometimes be determined. A more refined diagnosis can be achieved by examining wafers at various stages of processing. This can exclude some possible sources of defects from further consideration. Approaches that evaluate a system based on the results on a wafer are considered "exemplar" based.
One problem with these exemplar approaches is that it is difficult to deconvolve the effects of the different aspects of processing. For example, a defect may be due to anomalies in the exposure pattern, problems with a deposition, an etch, an implant, a chemical reaction, etc. A more complete diagnosis can be achieved by looking at the effects of one of these factors in isolation.
The photolithographic pattern lends itself to such an isolated analysis. The exposure can be evaluated independently of its effects in combination with other processing aspects. If no exposure defects are found, the defects can be assumed to be due to another source. Detailed examination of other processing aspects can then be investigated with greater confidence. If exposure defects are found, these can be corrected prior to new evaluation runs. The new evaluation runs can be used to find additional defect sources.
To take advantage of computing power to analyze images, a digital image is desired. Direct digital images are most desirable. However, devices such as charge-coupled devices (CCDs) that would otherwise be suitable, are considerable thicker than most wafers. When a CCD is placed on the wafer stage used for successive stepping the integrated circuits for exposure, the surface of the CCD is significantly above the typical wafer plane. Hence, the CCD would detect different and relatively unfocused pattern that would be difficult to compare to the pattern actually imposed on a wafer. Changing beam focus can provide a sharper image, but conditions would then vary from those to which the wafer is subjected. Theoretically, the stage on which wafers are mounted could be lowered to allow the CCD to match the wafer plane. However, it can be impracticable to recover precisely the original stage settings. In general, such disturbances of processing equipment are considered unacceptably intrusive.
An alternative digitizing approach uses a single optical fiber to interrogate a beam pattern pixel-by-pixel. The fiber can be mounted in a frame that can lie on the wafer stage. The fiber can extend horizontally except at one end that is turned upward. Optical fibers are sufficiently flexible that the upward facing end can be at the wafer plane. The frame, and thus the upward facing fiber end, can be raster scanned through the illumination field. The intensity of light exiting the opposite end of the fiber can be measured and recorded. The recorded data can be assembled into a digital image.
A major problem with fiber optic sampling is the limited resolution given available fiber cross sections. Another problem is that the exposure required for serial interrogation is much longer than that typically applied to a wafer, making the actual and test situations somewhat incomparable. In addition, there can be a problem assembling the serially acquired image precisely, especially since jitter can cause registration to vary during the relatively long exposure. Furthermore, it is undesirable to operate the large, expensive and power hungry light sources used in photolithography for such long exposures.
To avoid the long exposure required for serial digitizing, an exposure pattern can be captured on film; the film image can then be digitized "off line" as it was. Conventional films require development that can introduce artifacts in the image. One then has the problem of deconvolving these artifacts from the exposure pattern. This is analogous to the problem with exemplar evaluation. If standard photoresist is used as the film, the problems are more than analogous. In general, conventional films are undesirable for evaluation since the development time adds to the down time of a costly fabrication system.
"Super resolution films" are known that provide "direct" images without development. Such films have been used to evaluate photolithographic masks. However, while the exposures required for the use of such films are short relative to serial digital acquisition, they are long relative to the time photoresists are exposed. Accordingly, there is loss of time-varying information and vulnerability to jitter. To minimize the effects of jitter, a contact print can be made of the mask. In this case, however, the image is acquired far from the wafer plane, which is undesirable.
None of the methods discussed above has been used successfully to evaluate exposure patterns near the plane of the wafer. Grosser evaluations of intensity distributions have been made during a period when a mask is removed. Reports have been made of relatively poor quality mask projections taken away from the wafer plane. None of the methods has come close to providing means for evaluating the uniformity of line widths in an exposure pattern. Thus, it has not been possible to evaluate contributions of mask errors to line width variations and the myriad of defects and reliability problems with which they can be associated.
What is needed is a method for evaluating exposure pattern line widths. Preferably, such a method would acquire images over short durations and on the wafer plane. Furthermore, the method should be compatible with and minimally disruptive of wafer fabrication systems.